Cold-rolled steel sheet for flux-cored wire and method for producing same

This application is the U.S. National Phase under 35 U.S.C. § 371 of International Patent Application No. PCT/KR2020/018457, filed on Dec. 16, 2020, which in turn claims the benefit of Korean Application No. 10-2019-0171743, filed on Dec. 20, 2019, the entire disclosures of which applications are incorporated by reference herein.

The present invention relates to a cold-rolled steel sheet for a flux-cored wire and a method for manufacturing the same. More specifically, the present embodiments relate to a cold-rolled steel sheet for a flux-cored wire in which strength, low-temperature toughness, welding workability and workability characteristics are remarkably improved by optimizing the content of alloying elements, and a method for manufacturing the same.

In general, there is a flux cored welding (FCW) method as a welding method in which welding productivity is highest and welding is easily performed at various positions. A welding material used in the FCW method is a flux-cored wire, and is prepared by processing a strip obtained by drawing a cold-rolled steel sheet for a welding rod into a U-shape, and then adding flux to the processed U-shaped tube.

A carbon steel-based cold-rolled steel sheet is typically used as a cold-rolled steel sheet for a flux-cored wire used in the manufacture of such a flux-cored wire, and stainless steel is used for some special uses.

Since a carbon steel-based cold-rolled steel sheet for a flux cored wire is low-alloy steel, in addition to a basic flux component filled inside the core, a large amount of alloying elements for securing the usage characteristics need to be added in order to secure the characteristics of the flux cored wire depending on the use environment.

However, when the content of the alloying element for securing the use characteristics of the welding rod is increased as described above, there is a problem in that it is difficult to secure stable welding characteristics because the flux component and the like are limited. In addition, as most of these alloying elements are added in the form of a high-purity powder, these added alloying elements not only cause an increase in cost, but also have high specific gravities, so that there is a problem in that the additive components melted during welding act as a factor of welding defect such as causing segregation of a welded portion.

For example, as one of the methods for manufacturing a steel sheet for a flux-cored wire, a method for manufacturing a steel for a welding rod having excellent impact toughness and strength by adding titanium (Ti) or the like has been presented. However, the method has a problem in that the manufacturing cost increases as a large amount of expensive alloying elements are added, and also has a problem in that it is difficult to secure the drawability due to low ductility.

Further, a technique has been proposed, in which welding defects are reduced by adding titanium (Ti), magnesium (Mg) and the like to a flux raw material to promote the deoxidation reaction of a molten metal. However, many alloying elements need to be added to the flux in order to sufficiently obtain the deoxidizing effect of the molten metal, but there is a problem in that when such a large number of alloying elements are added to the flux, welding workability deteriorates, such as occurrence of many spatter phenomena in which fine particles are ejected to the surrounding area during welding.

Therefore, there is a need for developing a welded steel strip using a cold-rolled steel sheet for a flux-cored wire, which is capable of obtaining a welded portion having excellent strength and low-temperature toughness and has excellent welding workability and drawability in an extremely low-temperature environment, and a method for manufacturing the same.

The present embodiment has been made in an effort to provide a cold-rolled steel sheet for a flux-cored wire, in which strength, low-temperature toughness, welding workability and workability are remarkably improved by adding a suitable amount of nickel (Ni), boron (B) and the like, and a method for manufacturing the same.

A cold-rolled steel sheet for a flux-cored wire according to an exemplary embodiment may include: by wt %, 0.0005 to 0.01% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.03% or less (except for 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.001 to 0.008% of sulfur (S), 0.0001 to 0.010% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.5 to 1.7% of nickel (Ni), 0.0005 to 0.0030% of boron (B), and the balance Fe and inevitable impurities.

A method for manufacturing a cold-rolled steel sheet for a flux-cored wire according to another exemplary embodiment may include: manufacturing a slab including: by wt %, 0.0005 to 0.01% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.03% or less (except for 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.001 to 0.008% of sulfur (S), 0.0001 to 0.010% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.5 to 1.7% of nickel (Ni), 0.0005 to 0.0030% of boron (B), and the balance Fe and inevitable impurities, heating the slab, obtaining a hot-rolled steel sheet by hot rolling the heated slab such that a finishing hot rolling temperature is 890 to 950° C., winding the hot-rolled steel sheet in a temperature range of 550 to 700° C., obtaining a cold-rolled steel sheet by cold rolling the wound hot-rolled steel sheet at a rolling reduction ratio of 50 to 85%, and annealing the cold-rolled steel sheet in a temperature range of 700 to 850° C.

Since the cold-rolled steel sheet for a flux-cored wire according to an exemplary embodiment remarkably improves workability and productivity by appropriately controlling the alloy component and simultaneously, easily secures welding workability by stabilizing a flux component, the efficiency of work can be dramatically improved.

Further, according to an exemplary embodiment, the cold-rolled steel sheet for a flux-cored wire used in the shipbuilding industry, the materials industry, the construction industry, and the like can be manufactured at low cost.

Terms such as first, second and third are used to describe various parts, components, regions, layers and/or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, a first part, component, region, layer, or section to be described below could be termed a second part, component, region, layer, or section within a range not departing from the scope of the present invention.

The terminology used herein is solely for reference to specific exemplary embodiments and is not intended to limit the present invention. The singular forms used herein also include the plural forms unless the phrases do not express the opposite meaning explicitly. As used herein, the meaning of “include” specifies a specific feature, region, integer, step, action, element and/or component, and does not exclude the presence or addition of another feature, region, integer, step, action, element, and/or component.

When a part is referred to as being “above” or “on” another part, it may be directly above or on another part or may be accompanied by another part therebetween. In contrast, when one part is referred to as being “directly above” another part, no other part is interposed therebetween.

Although not differently defined, all terms including technical terms and scientific terms used herein have the same meaning as the meaning that is generally understood by a person with ordinary skill in the art to which the present invention pertains. The terms defined in generally used dictionaries are additionally interpreted to have the meaning matched with the related art document and currently disclosed contents, and are not interpreted to have an ideal meaning or a very formal meaning as long as the terms are not defined.

Further, unless otherwise specified, % means wt %, and 1 ppm is 0.0001 wt %.

In an exemplary embodiment of the present invention, further including an additional element means that the additional element is included while replacing iron (Fe) that is the balance by an additional amount of the additional element.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings such that a person with ordinary skill in the art to which the present invention pertains can easily carry out the present invention. However, the present invention may be implemented in various different forms, and is not limited to the exemplary embodiments described herein.

A cold-rolled steel sheet for a flux-cored wire according to an exemplary embodiment of the present invention may include: by wt %, 0.0005 to 0.01% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.03% or less (except for 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.001 to 0.008% of sulfur (S), 0.0001 to 0.010% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.5 to 1.7% of nickel (Ni), 0.0005 to 0.0030% of boron (B), and the balance Fe and inevitable impurities.

Hereinafter, the reasons for limiting the components of the cold-rolled steel sheet will be described.

C: 0.0005 to 0.01 wt %

Carbon (C) is an element added to improve the strength of steel, and is an element added to make a welding heat affected zone have characteristics similar to those of a base material. When the content of C is too low, the above-described effects are insufficient. In contrast, when the content of C is too high, problems such as disconnection may occur during a drawing process due to high strength or work hardening. Further, there are disadvantages in that a cold-rolled steel sheet can be processed into a target final product only when a plurality of heat treatments is performed because not only low-temperature cracks occur in welded joints or impact toughness decreases, but also hardness is high. Accordingly, the content of C may be 0.0005 to 0.01 wt %. More specifically, for example, the content of carbon (C) may be in a range of 0.0005 to 0.008 wt %, 0.0005 to 0.005 wt %, or 0.001 to 0.005 wt %. When the carbon content satisfies the above range, the characteristics of the welding heat affected zone may be further improved.

Mn: 0.05 to 0.25 wt %

Manganese (Mn) is a solid solution strengthening element, and serves to increase the strength of steel and improve hot workability. However, when manganese is added in an excessive amount, the ductility and workability of a steel may be inhibited by forming a large amount of manganese-sulfide (MnS) precipitates. When the content of Mn is too low, it may cause red shortness and it may be difficult to contribute to the stabilization of austenite. In contrast, when the content of Mn is too high, the ductility deteriorates and the too high content of Mn acts as a factor for the occurrence of center segregation, so that it is possible to induce disconnection during the drawing work in a process of manufacturing a welding rod. Accordingly, the content of Mn may be 0.05 to 0.25 wt %. More specifically, for example, the content of Mn may be 0.07 to 0.20 wt %.

Si: 0.03 wt % or Less

Silicon (Si) not only may serve as a factor which degrades the surface characteristics and reduce corrosion resistance by combining with oxygen to form an oxide layer on the surface of a steel sheet, but also serves as a factor which degrades low-temperature impact characteristics by promoting a hard phase transformation in the weld metal. Accordingly, the content of Si is limited to 0.03 wt % or less. More specifically, for example, the content of Si may be 0.001 to 0.030 wt % or 0.001 to 0.0020 wt %.

P: 0.0005 to 0.01 wt %

Phosphorus (P) is an element which improves strength and hardness by causing solid solution strengthening while being present as a solid solution element in steel. When the content of P is too low, it may be difficult to maintain a certain level of rigidity. When the content of P is too high, center segregation occurs and ductility deteriorates during casting, so that the wire workability may be inferior. Accordingly, the content of P may be 0.0005 to 0.01 wt %. More specifically, for example, the content of P may be 0.001 to 0.008 wt %.

S: 0.001 to 0.008 wt %

Since sulfur (S) combines with manganese in the steel to form non-metal inclusions and causes red shortness, it is desirable to lower the content thereof as much as possible. In addition, when the content of S is too high, there may be a problem of reducing the toughness of the base material of the steel sheet. Accordingly, the content of S may be 0.001 to 0.008 wt %. More specifically, for example, the content of S may be 0.0015 to 0.007 wt %.

Al: 0.0001 to 0.010 wt %

Aluminum (Al) is an element added for the purpose of preventing a material from deteriorating by a deoxidizer and aging in an aluminum killed steel, and is also advantageous for securing ductility, and such an effect is more remarkable at extremely low temperature. When the content of Al is too low, the above-described effects are insufficient. In contrast, when the content of Al is too high, surface inclusions such as aluminum-oxide (Al2O3) are rapidly increased to cause the surface characteristics of a hot-rolled material to deteriorate, and not only the workability deteriorates, but also ferrite is locally formed at the crystal grain boundary of the welding heat affected zone, so that mechanical characteristics may deteriorate. Furthermore, there may be a problem in that the shape of the weld bead deteriorates after welding. Accordingly, the content of Al may be 0.0001 to 0.010 wt %. More specifically, for example, the content of Al may be 0.0005 to 0.0100 wt %, 0.001 to 0.007 wt % or 0.001 to 0.006 wt %.

N: 0.0005 to 0.003 wt %

Nitrogen (N) is an element that is effective for strengthening materials while being present in a solid solution state in steel. When N is included in too small an amount, it may be difficult to secure the target rigidity. In contrast, when the content of N is too high, not only the aging properties deteriorates sharply, but also the burden due to denitrification increases in the steel manufacturing step, and the steelmaking workability may deteriorate. Accordingly, the content of N may be 0.0005 to 0.003 wt %. More specifically, for example, the content of N may be 0.001 to 0.0027 wt %.

Ni: 0.5 to 1.7 wt %

Nickel (Ni) is an element which is not only effective for improving the drawability by improving the ductility, but also required to improve low-temperature impact characteristics by forming a stable structure even at extremely low temperature. In order to obtain the aforementioned effects and simultaneously stably control the flux composition, Ni may be included in an amount of 0.5 wt % or more. However, when the content of Ni is too high, not only the drawability deteriorates due to an increase in strength, but also surface defects may be induced. Further, when a large amount of fundamentally expensive Ni is added, the steelmaking cost may remarkably increase. Accordingly, the content of Ni may be 0.5 to 1.7 wt %. More specifically, for example, the content of Ni may be 0.5 to 1.6 wt %, 0.6 to 1.6 wt %, or 0.7 to 1.5 wt %.

B: 0.0005 to 0.0030 wt %

Boron (B) is an element that is advantageous in terms of securing the strength of welded joints by enhancing hardenability. When B is included in too small an amount, it may be difficult to secure strength. In contrast, when B is included in too large an amount, there may occur a problem in that by increasing the recrystallization temperature, not only the annealing workability is lowered, but also the workability is remarkably lowered. Accordingly, the content of B may be 0.0005 to 0.0030 wt %. More specifically, for example, the content of B may be 0.0002 to 0.004 wt %, 0.0005 to 0.0030 wt %, 0.0006 to 0.0027 wt % or 0.001 to 0.0027 wt %.

The remaining component of the present invention is iron (Fe). However, since unintended impurities may be inevitably incorporated from the raw material or the surrounding environment in the typical manufacturing process, these impurities cannot be excluded. Since these impurities are known to a person with ordinary skill in the typical manufacturing process, all the contents thereof are not specifically mentioned in the present specification.

Meanwhile, the cold-rolled steel sheet of the present invention not only satisfies the above-described alloy composition, but also may have W,of 2.0 to 15.0, which is defined by the following Equation 1.W,=(41×[C]+28×[Al]+3.4×[S])*(25×[Ni]×30×[B])/(25×[N])  [Equation 1]

In Equation 1, [C], [Al], [S], [Ni], [B] and [N] indicate the contents (wt %) of C, Al, S, Ni, B and N, respectively.

W,is designed in consideration of the correlation of each element on welding workability and drawability. When W,is too small, the degree of hardening of a welded portion structure is low, so that the workability is improved, but the welding strength and low-temperature toughness cannot be secured, so that the amount of alloying elements in the flux needs to be increased. Accordingly, the welding workability may deteriorate. In contrast, when W,is too large, the hardness of the welded portion increases sharply, so that a welded member may break during the pipe making and drawing work. Therefore, it is preferred that W,satisfies a range of 2.0 to 15.0. More specifically, for example, W,may be in a range of 2.1 to 14.8.

The cold-rolled steel sheet according to an exemplary embodiment of the present invention has excellent elongation. Specifically, the elongation may be 40% or more. By satisfying such physical properties, the cold-rolled steel sheet may be preferably applied as a material for a flux-cored wire.

Specifically, a flux-cored wire is manufactured by continuously passing a cold-rolled steel sheet strip between rolls to increase the amount of bending deformation, then molding the steel sheet into a U-shaped bent member, and then supplying flux to the inside of the member. Thereafter, the flux-filled material is again allowed to continuously pass between rolls to produce a cylindrical shape whose inside is filled with flux, and a flux-cored wire is manufactured in the form of being drawn to a desired thickness by again pulling out the material in the longitudinal direction. Therefore, a high elongation is required in order to be applied as a material for a flux-cored wire.

When the elongation is too low, the reduction rate of the cross section is lowered during a drawing processing of a welding wire, so that there may be a problem in that pipe-making workability deteriorates and cracks such as tears occur during the processing. More specifically, the cold-rolled steel sheet according to an exemplary embodiment may have an elongation of 40% to 60%, 44% to 55%, or 45% to 55%.

The method for manufacturing a cold-rolled steel sheet for a flux-cored wire according to an exemplary embodiment of the present invention may include manufacturing a slab, heating the slab, obtaining a hot-rolled steel sheet by rolling the heated slab, winding the hot-rolled steel sheet, obtaining a cold-rolled steel sheet by cold rolling the wound hot-rolled steel sheet, and annealing the cold-rolled steel sheet.

Hereinafter, the method will be specifically described for each step.